Why my store orders have disappeared?

All orders are saved in published website in one of the data file. The only reason of missing orders may be related to file deletion (possibly you deleted website files and then re-published website).

The older file having all orders may be returned by restoring published website backup from hosting control panel of earlier version. After restoring the client should check in website builder if earlier orders appeared. However, note that website backup restoring on hosting will restore the whole earlier website version. So, to make it up-to-date again just re-publish website in website builder (website builder will have up-to-date version). The re-publication will not delete restored orders.